Too far

"You're assisting in a violent crime and I am not going to take two seconds to consider "Is it excessive force to cap the one who's not actively humping?"

According to the laws of both of our nations, you're toeing an extremely dangerous line between reasonable self-defense and murder.

If you don't even stop to consider it, how do you know you're right? Are sledgehammers appropriate? Flamethrowers? Maybe a howitzer? How about caltrops, to stop them from getting away? Speaking of getting away...

"If I can stop him from getting away and it takes a bullet to do it, that's just what it takes."

That's just vigilanteism. It is, and should be, outright illegal. You have no rights, regardless of another person's criminality, to step beyond the bounds of the reasonable defense of yourself and others. You can't shoot criminals to prevent them from escaping just as you can't don a cape, mask and uzi and fight for the cause of justice.

If your ethical system dictates that it is appropriate, despite it being murder, to shoot someone in that situation, then I suppose you'd just have to accept the concequences. I see no reason for society to change its mind about throwing you in prison for excessive use of force.